It's up to you = It's your choice Example: Sarah: Hey Mark, what movie do you want to watch tonight, Titanic or Star Wars? Mark: I'm fine with either so it's up to you. Sarah: Ok, then I choose Titanic! Go with the flow = To go with the flow is to let things happen the way they're happening. The opposite of "go with the flow" would be to "swim against the current". Example: Imagine floating on your back in the ocean and just letting the tide carry you wherever it wants to carry you. You are relaxed and don't fight the waves that are moving you. You are going with the flow. I hope that helped! Feel free to message me if you still have some confusion or want further elaboration. :)